The acetylcholine (ACh) receptor in vertebrate skeletal muscle is normally confined to the endplate membrane, but during development and after denervation, ACh receptors also occur over the entire surface of the cell. Our experiments are aimed at understanding the properties of the receptors in these two locations; how their concentrations are maintained and changed under different physiological and developmental conditions; and what their respective physiological roles are. In previous investigations, we have demonstrated that after denervation, newly synthesized receptor is accumulated, and is presumably responsible for the large increase in levels of extrajunctionl receptor that occurs. Since there is considerable evidence that muscle activity controls extrajunctional receptor levels, we propose to study the effects of direct electrical stimulation on receptor synthesis in organ culture. We will also examine rates of receptor degradation by measuring the breakdown of labelled receptor. The results will be compared with those obtained earlier by an indirect method, in which loss of radioactive toxin bound to receptor in situ was measured. Work from our laboratory and others has shown that junctional and extrajunctional ACh receptors are molecules that are highly similar, but can be distinguished by pharmacological, physical, and immunological criteria. Neither the significance of this difference nor the functional role of extrajunctional receptors is understood. We propose to investigate further the structural differences between junctional and extrajunctional receptors. We have suggested that extrajunctional receptors may play a role in synapse formation and will investigate the appearance of junctional receptors during formation of new endplates in adult muscle and in cell culture. To further investigate these problems, animals will be immunized against the ACh receptor, and the effects of this immunization on receptor accumulation and turnover in normal and denervated muscles examined. The ability of denervated muscles in these animals to become reinnervated will also be investigated.